Introduction: Cerebral venous sinus thrombosis (CVST) is a rare but serious neurological emergency in children. Clinical manifestations are often nonspecific, and recognised precipitating factors include dehydration, infection and prothrombotic states. Early diagnosis and prompt anticoagulation are essential to minimise neurological sequelae. Case: A previously healthy 4-year-7-month-old girl presented with a three-day history of fever, vomiting and watery diarrhoea, followed by focal seizures and reduced consciousness. On admission, she was hypotensive and clinically dehydrated with hypernatremia (serum sodium 154 mmol/L) and managed as septic shock. She required intubation and intensive care support. Blood cultures were sterile, cerebrospinal fluid analysis was normal, and computed tomography showed mild cerebral oedema. During recovery , she developed new behavioural changes. Although a post-infectious autoimmune process was considered, persistent neurological concerns prompted further evaluation. Magnetic resonance imaging with venography revealed extensive acute thrombosis of the bilateral transverse and sigmoid sinuses, jugular bulb and proximal internal jugular vein, without infarction or haemorrhage. She was treated with gradual correction of hypernatremia, broad-spectrum antibiotics and therapeutic anticoagulation with low-molecular-weight heparin, later bridged to warfarin (target INR 2–3). Gradual neurological improvement was observed, and behavioural symptoms improved on follow-up. Conclusion: This case highlights hypernatremic dehydration as an important reversible risk factor for paediatric CVST and underscores the need for early neuroimaging in children with evolving neurobehavioural changes after acute systemic illness.